function enp_append_popular_posts($content)
{
    if (is_single() || is_singular()) {
        // check the settings
        $enp_append_popular_slugs = get_option('enp_display_popular_slugs');
        if (!empty($enp_append_popular_slugs)) {
            foreach ($enp_append_popular_slugs as $slug) {
                $posts = enp_get_popular_posts($slug);
                $content .= $posts->popular_loop(5);
            }
        }
    }
    return $content;
}
function enp_popular_posts_HTML($atts, $filter_label = '')
{
    $posts = enp_get_popular_posts($atts['slug'], $atts['type']);
    return $posts->popular_loop($atts['how-many'], $filter_label);
}